Why did Virginia agree to go “slumming”with her old,fashionable friends?

The girls mean that they will "lower" themselves to visit the rectangle instead of going to the band concert.

"I tell you, girls, let's go 'slumming' with Virginia this afternoon instead of going to the band concert. I've never been down to the Rectangle. I've heard it's an awful wicked place and lots to see. Virginia will act as guide, and it would be real," -- "fun" she was going to say, but Virginia's look made her substitute the word "interesting."